It is frequently desirable to reinforce rubber articles, for example, tires, conveyor belts, power transmission belts, timing belts, hoses, and the like products, by incorporating therein steel cables. Pneumatic vehicle tires are often reinforced with cords prepared from brass coated steel filaments. Such tire cords are frequently composed of high carbon steel or high carbon steel coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments which are stranded together. In most instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord.
In order for rubber articles which are reinforced with steel wire elements to function effectively it is imperative that good adhesion between the rubber and the steel cord be maintained. Thus, generally steel wire reinforcement elements are coated with brass in order to facilitate rubber-metal adhesion.
It is generally agreed by those skilled in the art that adhesion of rubber to brass-plated steel wire is dependent upon a bond between the copper in the brass and sulfur in the rubber. When such brass coated steel reinforcing elements are present in the rubber composition during vulcanization, it is believed that bonds between the rubber and steel reinforcement form due to a chemical reaction between the brass alloy and the rubber at the interface forming a bonding interphase. The brass coating also serves an important function as a lubricant during final wet drawing of steel filaments.
Cables for reinforcing rubber articles which are comprised of predominantly alpha-brass coated filaments typically provide adequate original metal to rubber adhesion after being cured. However, the level of rubber to metal adhesion displayed diminishes with the passage of time. The presence of salt and humidity are known to have an adverse effect on rubber to metal adhesion. Tires as well as other reinforced rubber articles are often subjected to a long service life under conditions where salt and humidity are present in their operating environment. For this reason, it would be desirable to improve the aged rubber to metal adhesion, fatigue resistance and corrosion resistance of cables for reinforcing rubber articles.